The present invention relates to an improved hydraulic suspension strut for use in a wheeled vehicle.
Known hydraulic suspension struts comprise a piston movable within a cylinder to achieve a damper effect, allowing the wheel mounted on the strut to move in the vertical direction. The damping effect is achieved by restricting the flow of hydraulic fluid from one side of the piston to the other. Often the hydraulic strut will be connected to a gas charged hydraulic accumulator to achieve a combined damper and spring effect in which the spring effect is achieved by compression of hydraulic fluid in the cylinder and fluid in the gas charged hydraulic accumulator. It is also known to provide a hydraulic bump stop in a hydraulic suspension system of this type, in which a volume of hydraulic fluid is contained within the cylinder and limits movement of the piston and hence the wheel, to the distance normal for road travel.
It is also known for road vehicles to use hydraulic suspensions comprising various valves and cross-connections between individual wheel suspensions, for example the BMC/British Leyland Hydrolastic and Hydragas systems. Such interconnected suspensions are disclosed in GB 2,144,378 (Alfa Romeo), GB 1,260,719 (IDCE), and U.S. Pat. No. 5,584,498 (Yamaha). These systems are designed to reduce undesirable effects such as pitch and roll; as are active and semi-active systems, which are known, for example, from U.S. Pat. No. 4,779,895 (Robert Bosch), EP 0,183,059 (Robert Bosch), and U.S. Pat. No. 5,678,846 (Lotus). All of these systems are designed to improve on-road performance, by extending the capabilities of a road car suspension.
It is further known, particularly on Citröen cars, for a road vehicle to have hydraulic suspension which can be raised above normal running height to traverse uneven ground. On the other hand, for amphibious vehicles, it has been found helpful to move the wheels in the opposite direction, retracting them well above road height to tuck them above the vehicle water line. This reduces drag on water, particularly during cornering. Suspensions allowing such reconfiguration are disclosed in EP 0,742,761 (Roycroft), U.S. Pat. No. 4,958,584 (Williamson), and U.S. Pat. No. 4,241,686 (Westphalen). Roycroft uses a combined hydraulic and mechanical system; whereas both Williamson and Westphalen use mechanical systems.